The present invention is directed to a method and an outage recognition circuit or device for use with a redundantly designed optical transmission system which has each subscriber station provided with at least two receiver circuits with each receiver circuit having a separate transmission channel for carrying information to the subscriber station.
In order to increase the reliability in optical transmission systems, the systems are often redundantly designed so that the information will proceed to a subscriber station on more than one channel. When the geometric paths or routes of the redundant transmission channels are different in length, problems arise because the mutually corresponding bits in the data stream arrive at the subscriber station at different times. As a result thereof, the two data streams cannot be compared to one another without further ado because of the statistical phase position in order to derive a criterion for the outage of one or more channels.
A bit-by-bit comparison of the two data streams has heretofore been achieved by means of equalization of the different optical paths by means of inserting corresponding fiber lengths or by means of a matched, electrical delay in one of the receiver circuits so that corresponding bits chronologically coincide. In both solutions, the equalization measures must be individually executed for each station. On the other hand, the solutions make a very high and critical demand on the synchronization of the data bits and on the stability of the circuits of the system.